The present invention relates to a surveying instrument employing a sighting telescope having an auto-focusing function.
Conventionally, a surveying instrument such as an electric distance meter, an electronic theodolite, and the like have been known. The distance meter is for measuring a distance to an aiming point, and the theodolite, or an angle measuring instrument is for measuring horizontal and vertical (elevational) angles between a reference point and an aiming point.
In such a surveying instrument, a sighting telescope is provided for accurately sighting the instrument on an aiming point. By aligning the sighting telescope so that the aiming point is located at the center of field of view, the axis of distance measuring or measuring the horizontal and elevational angles is aligned to intersect the aiming point.
In order to view the aiming point clearly within the field of view of the sighting telescope, a focusing mechanism is provided. The focusing mechanism is to be manipulated so that an in-focus image of the aiming point is formed. To ease the focusing operation, recently, auto-focusing system have been employed in the sighting telescopes.
With the auto-focusing function, the focusing condition of the sighting telescope is automatically adjusted. Thus, the auto-focusing function makes the operator free from troublesome manipulation of focusing mechanism, and allows the operator to concentrate on centering the aiming point in the field of view of the sighting telescope.
In such a sighting telescope, when surveying is performed, a reflector is located at the aiming point. Generally, a corner cube is used as the reflector. The corner cube reflects an incident ray of light along the same direction thereof. Accordingly, when an operator sights the sighting telescope on the corner cube, and the optical axis of the sighting telescope intersects a reflection surface of the corner cube, the auto-focusing system focuses on a virtual image of the telescope formed by the corner cube. That is, the auto-focusing system focuses on an object (i.e., the telescope) located at twice a distance of the corner cube. Then, the corner cube in the field of view is fuzzy (out-of-focus), and the operator has to manipulate the focusing system, even if the auto-focusing system is employed, to focus on the corner cube.